Sulfite, oxidation of sodium

Mass-transfer rates have been determined by measuring the absorption rate of a pure gas or of a component of a gas mixture as a function of the several operating variables involved. The basic requirement of the evaluation method is that the rate step for the physical absorption should be controlling, not the chemical reaction rate. The experimental method that has gained the widest acceptance involves the oxidation of sodium sulfite, although in some of the more recent work, the rate of carbon dioxide absorption in various media has been used to determine mass-transfer rates and interfacial areas. 

Measurements Using Liquid-Phase Reactions. Liquid-phase reactions, and the oxidation of sodium sulfite to sodium sulfate in particular, are sometimes used to determine kiAi. As for the transient method, the system is batch with respect to the liquid phase. Pure oxygen is sparged into the vessel. A pseudo-steady-state results. There is no gas outlet, and the inlet flow rate is adjusted so that the vessel pressure remains constant. Under these circumstances, the inlet flow rate equals the mass transfer rate. Equations (11.5) and (11.12) are combined to give a particularly simple result ... 

Figure 10.9. Typical data of mass transfer coefficients at various power levels and superficial gas rates for oxidation of sodium sulfite in aqueous solution. d/D = 0.25-0.40 (Oldshue, 1983).

The sodium sulfite oxidation method (Cooper et al., 1944) is based on the oxidation of sodium sulfite to sodium sulfate in the presence of catalyst (Cu++ or Co++) as... 

Air oxidation of sodium sulfite solutions, containing a trace of cupric ion as catalyst, was first studied by Cooper et al. (C7). These workers found the reaction rate to be essentially independent of sulfite and sulfate concentrations and therefore especially convenient for their study. Most of their runs were made using a vaned-disk agitator (a flat disk with sixteen radial vanes on its lower face) in baffled vessels 6-17 in. in diameter. Tank-to-impeller diameter ratio was 2.5, and the impeller was 0.3 tank diameter off the bottom, in every case. Impeller speeds were 60-900 r.p.m. A few measurements were made with a simple flat paddle, with a total length one-fourth the vessel diameter, in 9.5- and 96-in. diameter tanks. Air was introduced from a single open-end pipe just below the center of the agitator. Superficial air velocities were 0.3-11 ft./min. 

Oxidation of sodium sulfite with cobaltous sulfate catalyst... 

C02-N2 mixture and water Oxidation of sodium sulfite, C02-water... 

The kinetics of the Co + heterogeneously catalysed oxidation of sodium sulfite were measured by Sawicki and Barron [480] in a falling-film column. In contrast with [568] the reaction imder such conditions was found to be of second-order with respect to oxygen concentration and 0.5-order with respect to Co + concentration. 

Since Backstrom (.1,2) reported studies of the thermal and photo-oxidation of sodium sulfite solutions, the oxidation of S(IV) species by oxygen has been known to proceed by a chain mechanism. The details of the mechanism, however, are still a matter of much controversy. 

There are several organic compounds which are known to inhibit the aqueous phase oxidation of sulfite (.7, 8, 9, 10). Several metals are known to have a catalytic effect on the rate of oxidation. Fuller and Christ (10) demonstrated that copper sulfate is an effective catalyst. Cobalt also catalyzes the oxidation of sodium sulfite (11). 

The presence of sodium sulfate and sodium chloride is principally the result of secondary absorption reactions. Sodium sulfate is formed by the oxidation of sodium sulfite via reaction with oxygen absorbed from the flue gas. Oxidation also occurs in other parts of the system where process solutions are exposed to air however, the amount of oxidation is small relative to the oxidation which occurs in the absorber. At steady state, the sulfate must leave the system either as calcium sulfate or as a purge of sodium sulfate at the rate at which it is being formed in the system. Although a practical limit for the level of oxidation that can be tolerated by the limestone dual alkali system has not yet been established, it appears that oxidation rates equivalent to 15 to 20% of the S02 removed might be accommodated without intentional purges of sodium sulfate. 

Many correlations have been suggested to estimate mass transfer coefficients in two-phase systems for various types of physical scenarios. Oldshue (1983) gives a summary of available correlations for liquid-solid, liquid-liquid, and gas-liquid mass transfer, as well as an estimate of the mass transfer rate of a G-L-S system—namely, the oxidation of sodium sulfite in water. Cussler (1997) gives... 

In order to estimate the efficiency of the mass transfer in gas-liquid flows, a sulfite method [22], based on the catalytic oxidation of sodium sulfite by atmospheric oxygen, was used ... 

Production of acetaldehyde from oxidation of C H. In a solution of CuCU containing PdCU as a catalyst Liquid-phase esterification of terephthallc acTd lth methanol Hydrogenation of methyl llnoleate In the presence of a palladium catalyst Oxidation of sodium sulfite with cobaltous sulfate catalyst... 

According to Baier (2001), the ABLR yields the same mass transfer performance (defined as oxygen transfer rate obtained by cobalt-catalyzed oxidation of sodium sulfite) at half the power input as compared to the conventional BLR. 

EXAMPLE 2 The oxidation of sodium sulfite or benz-aldehyde is retarded by the addition of small amounts of alcohols such as mannitol or benzyl alcohol. 

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